Method and device for quenching steel in pressurized air

ABSTRACT

The method for quenching a steel charge after carburizing or carbonitriding is carried out at atmospheric pressure and comprises the following steps: the charge is extracted from a treatment furnace (1) at a temperature ranging from 750 ° C. to 1100 ° C.; b) the charge is transferred to a quenching cell (3); c) a quenching fluid is introduced at a pressure which is higher than atmospheric pressure; d) the charge is cooled to a temperature of less than 400 ° C. According to the invention, the fluid is primarily composed of air, and the part is brought to a temperature of at least 400 ° C., whereby an oxide layer preventing decarburization is formed in the time period starting from the moment when the charge exits ( 1 ) from the furnace.

The present invention concerns a quenching method for steel with air under pressure that permits guaranteeing metallurgical characteristics identical to quenching in oil and, in particular, to avoid surface decarburization after a carburizing or carbonitriding treatment carried out at atmospheric pressure.

The quenching methods under gas pressure present numerous advantages compared to methods of quenching with liquids, above all because the pieces are clean and dry after quenching.

Numerous articles were devoted to quenching of steel with gas under pressure and today there are several devices which are characterized by the architecture of the quenching cell and the nature of the gas used, the objective being to obtain as great a quenching efficacy as possible, that is, the fastest rate of cooling, in order to equal the rate of cooling obtained with liquids, such as oils, aqueous solution or molten salts. For this purpose, the mass flow rate of the circulating gas at the surface of the piece to be cooled is increased, by increasing its speed and its pressure.

The gas which is most currently used is nitrogen because it is chemically inert and its cost is relatively acceptable, so that it does not have to be recycled after each quenching operation. Most frequently, it is used under a pressure between 1 and 10 bar.

Other inert gases have been proposed. Thus, European Patent 0 313 888 proposes to use light gases, such as helium and hydrogen at elevated pressures (20 bar and higher). The utilization of these light gases has the effect that, at equal performances, the power of the engine of the turbine that circulates them can be limited. However, these gases are either particularly costly, which requires recycling, such as helium, or are particularly dangerous, such as hydrogen.

All gases proposed in the prior art are characterized by their physical characteristics: molecular weight, thermal conductivity, etc., but they are all chemically inert toward a steel brought to a high temperature before quenching. The objective is to avoid oxidation and surface decarburization of the steel.

The devices used to carry out quenching with a gas under pressure all involve a turbine that permits the circulation of the gas on the pieces. The gas is then cooled by passage over a heat exchanger and is then reinjected onto the charge of the pieces.

The Applicant set as goal the use of air under pressure to carry out the quenching because of its low cost and availability. However, its utilization poses a major problem. Oxygen is chemically very reactive toward steel at high temperature and usually causes surface decarburization, which is industrially unacceptable.

As a matter of fact, it is well known that keeping steel in air at a high temperature, in particular, between 700 and 110020 C., leads to the formation of oxide layers, frequently called “scale”, which grows very fast. Its thickness may reach several tens of micrometers after a few minutes of exposure to air. This formation of the oxide layer is also accompanied by decarburization of the steel surface, which is prejudicial to its metallurgical characteristics and its mechanical properties.

Therefore, the invention is concerned with a method and its embodiments, which permit conformance of quenching of steel after carburizing or carbonitriding treatment, carried out at atmospheric pressure with air under pressure from temperatures between 700 and 1100° C., such quenching leading to metallurgical characteristics, which are identical to those obtained after quenching in oil regarding the surface of the steel, that is, in particular, completely avoiding any surface decarburization.

According to the invention, the method of quenching of a steel charge comprises the following steps:

-   -   removal of the charge from a carburizing treatment furnace or         carbonitriding treatment furnace operating at atmospheric         pressure, at a temperature between 750° C. and 1100° C.,     -   transfer of the charge to a quenching cell,     -   introduction of a quenching fluid under pressure greater than         atmospheric pressure and circulating the fluid in the container,     -   cooling the charge to a temperature below 400° C., characterized         by the fact that the fluid is composed mainly of air, and that         the piece is brought to a temperature at most 400° C. during         such a time period after removal from the furnace that an oxide         layer is formed which prevents decarburization of the steel.

It is considered in the present description that decarburization of the steel did not occur when the carbon content measured over a thickness of 60 μm from the surface remains constant.

Thus, the method of the invention permits quenching of carbon steels, carburized steels, carbonitrided steels and tool steels under pressure in air.

The invention is based on the following observation. The oxidation of carbon steel at temperatures near 900° C. leads to the formation of a wustite-type oxide layer (sub-stoichiometric FeO). The thickness of this layer increases gradually with time to reach 10 to 12 μm after one minute in calm air. It is estimated that a layer of wustite oxide with a thickness of 1 μm is formed in air after 5 seconds. Its thickness is approximately 4 μm after 20 seconds at 870° C., which is the usual temperature of beginning of quenching of carburized or carbonitrided steels.

Moreover, it has been observed that, after 20 seconds and in the presence of an atmosphere, the carbon potential of which is less than 0.1% at 870° C., the layer affected by decarburization is of the order of 20 μm. This phenomenon of decarburization appears when the carbon monoxide layer can be released into the atmosphere. It results from the reaction ½O₂+C _(→)CO, where C is the carbon dissolved in the steel.

Thus, decarburization of the surface layers of the steel is avoided by forming a barrier which opposes the release of carbon oxide completely. Due to the very short time that is available, this barrier must form rapidly.

The invention recognized that the oxide layer of the wustite-type can play this role to the extent that it adheres sufficiently to the surface of the steel.

A thickness of 1 μm suffices to prevent the formation of CO in a significant manner.

In the absence of rapid variation of temperature (thermal shock), good adherence of the oxide layer is obtained when its thickness remains less than 10-12 μm and preferably when the thickness is less than 4 μm to 5 μm. Thus, near the usual quenching temperatures, one has a period of 20 to 40 seconds during which the iron oxide layer formed behaves as a barrier, opposing any release of carbon monoxide, which permits avoidance of any significant decarburization of the steel surface.

According to another characteristic of the invention, the charge consisting of steel pieces kept at the beginning temperature of the quenching is transferred into the quenching cell and the quenching begins by bringing the surface of the pieces to a temperature below 600° C. in a period of less than 40 seconds, preferably in less than 20 seconds.

The quenching fluid in the cell is under pressure that could reach 40 bar and is put into movement at the rate that can reach 20 m/s in a few seconds. The quenching fluid is composed essentially of air. Other constituents aimed at improving the heat transfer can be added to it.

The temperature of the surface of pieces was measured during the quenching. It drops to a value below 700° C. in less than 2 seconds, to a value below 600° C. in less than 4 seconds and to below 400° C. in less than 10 seconds. At a temperature below 600° C., the rate of diffusion of carbon in the steel is so slow that the decarburization phenomenon is no longer observed.

Other advantages and characteristics will follow from reading the description of a practical example, which is given below, and which does not limit the invention, accompanied by drawings on which:

FIG. 1 shows a schematic representation of a device according to the invention.

FIG. 2 is a graph showing the carbon concentration profile in the surface layers of the steel carburized and quenched according to the invention on the one hand and with oil on the other hand.

FIG. 3 is a graph showing the microhardness profile of the surface layers of carburized steels which were quenched according to the invention on the one hand, and in oil on the other hand.

The device of FIG. 1 involves a treatment furnace which itself is known.

It can be a carburization or carbonitriding furnace (1), which can be continuous- or batch-type, with a furnace well, a furnace bell, in which the thermochemical treatments are performed at atmospheric pressure.

On the same level at some distance, a quenching cell (3) is shown in which a gaseous fluid is used. This cell is closed hermetically with a door (31). Inside, a rotor provided with blades (33), for example, centrifugal or helicoidal blades, called turbine, is put in rotation with an electric motor (35). The function of the turbine is to put the gaseous quenching fluid contained in the container into motion. It is guided with guide organs (37) in the direction represented by the arrows or in the reverse direction. The gas passes through a heat exchanger system (39). To carry out the quenching, the charge represented by block A4 is placed in the cell, which is then closed hermetically, as shown with the dotted line. Then, the gas is introduced into the cell at the desired pressure, between 1 and 40 bar and the turbine is set in motion (33). Preferably, the pressure is greater than 3 bar and, in particular, depending on the nature of the pieces to be treated, it is between 5 and 20 bar. Appropriate deflectors inject the cold gas against the charge A4. After the heat had been removed from the charge, it is guided toward the turbine. It passes through the heat exchanger, where it is brought to the quenching temperature.

In the example of FIG. 1, the heat exchanger and the turbine are arranged inside the cell, but they can also be placed outside it.

The charge to be treated can consist, for example, of gears or shafts.

A handler (40) is placed between the furnace and the cell. Its function is to receive the charge from the furnace and placing it on the support of the cell. Various positions of the charge are represented, between A0, where it is still in the furnace, and A4, where it is on its support during the quenching.

The handler consists of a plateau (41) which can turn around a vertical axis and can be displaced in height. The plateau (41) has an arm (43) which can be displaced in a horizontal plane and can collect a charge to be treated. The arm (43) is mobile between an exterior position, where it collects or deposits the charge, and an interior position, where the charge is lodged in a protective bell (45).

The operational cycle is the following:

-   -   A charge is at A0 inside the furnace (1) and is at its treatment         temperature. The furnace door is opened.     -   Arm (43) is against the opening. The arm onto which the charge         will be placed is extended. The arm is retracted. The charge         comes into position Al under the bell (45).     -   The plateau pivots in order to place the charge in position A2,         against the support (31).     -   The arm which deposits the charge on support (31) in position A3         is extended.     -   The support is displaced until it enters the interior of the         cell. The charge arrives in position A4. Its temperature has not         significantly diminished since it left the furnace.     -   The quenching is started.

According to the invention, between the beginning of the cycle and the first phase of the quenching, which brings the surface temperature of the charge to 600° C., the time elapsed is less than 40 seconds; this is the time during which a layer of oxide is produced and forms a decarburization barrier on the charge. In particular, the manipulation permits transfer between the furnace and the cell in less than 30 seconds.

The pieces which underwent the quenching of the invention were examined. Metallurgical studies carried out on gears and shafts, which were carbonitrided and quenched in air under the conditions described above, showed the following:

-   -   The microhardness profiles are realized in the flank and at the         foot of the teeth are equivalent to those obtained after direct         quenching at 870° C. in oil. The graph in FIG. 3 shows that the         microhardness values, expressed in VICKERS units and measured at         different depths on a piece treated with air quenching on the         one hand, and on a piece treated with oil quenching on the other         hand, are almost identical.     -   The pieces quenched in air under pressure do not show any         decarburization. FIG. 2 shows the carbon concentration profile         obtained on a gear at the flank of a tooth and at the feet of         the tooth. The profiles are identical for quenching in oil and         quenching in air. They are characteristic of a layer carburized         and quenched without decarburization on the surface. A first         series of measurements is reported on a piece which underwent         carbonitriding treatment followed by quenching in air at 870° C.         A second series of measurements refers to a piece which         underwent carbonitriding treatment followed by quenching in oil.     -   Quenching in air under pressure causes the formation of an oxide         layer, the thickness of which is 6 μm.

The present invention also comprises a method in which the transfer is carried out under a protecting atmosphere and where the quenching is carried out with air at a pressure greater than atmospheric pressure. In this case, decarburization is essentially avoided during quenching by the formation of an oxide layer during the first phases of the quenching.

The present invention also covers a method where the quenching fluid is air to which a gas was added to modify its density and/or its thermal conductivity.

The present invention also covers a method where the quenching fluid is air to which an atomized liquid was added that permits cooling of the pieces with a two-phase mixture. 

1. Method of quenching of a steel charge after a treatment of carburization or carbonitriding, carried out at atmospheric pressure, comprising the following steps: a. the charge is removed from the treatment furnace (1) at a temperature between 750° C. and 1100° C., b. the charge is transferred to a quenching cell (3), c. a quenching fluid is introduced under a pressure greater than atmospheric pressure and it is put into circulation inside the cell, d. the charge is cooled to a temperature below 400° C., characterized by the fact that the fluid is composed mostly of air, and that the piece is brought to a temperature of at most 400° C. within a time from removal from the furnace (1) during which an oxide layer is formed which prevents decarburization of the steel.
 2. Method according to claim 1, characterized by the fact that the transfer is carried out with open air, and the temperature of the charge is lowered to 600° C. in less than forty seconds, preferably in less than twenty seconds.
 3. Method according to claim 1 or 2, characterized by the fact that the thickness of the oxide layer formed is less than 12 μm, preferably less than 5 μm.
 4. Method according to claim 1, characterized by the fact that the transfer is carried out under a protecting atmosphere.
 5. Method according to one of claims 1 to 4, characterized by the fact that the fluid is a mixture of air and of a gas, which modifies its density and/or its thermal conductivity.
 6. Method according to one of claims 1 to 4, characterized by the fact that the fluid is a two-phase mixture of air/atomized liquid.
 7. Method according to one of the preceding claims, in which the quenching fluid inside the cell is at a pressure between 3 and 40 bars, preferably between 5 and 20 bars.
 8. Device for carrying out the method according to one of the preceding claims, characterized by the fact that it comprises a transfer handler and a quenching cell capable of holding the charge between the furnace and the cell at a surface temperature of 600° C. in less than forty seconds.
 9. Device according to the preceding claim, characterized by the fact that the handler permits transfer in less than thirty seconds.
 10. Device according to claim 8 or 9, characterized by the fact that the quenching cell comprises a turbine which allows lowering of the temperature of the surface of the pieces of the charge to 400° C. in less than 10 seconds.
 11. Device according to one of claims 8 to 10, characterized by the fact that it is associated with a continuous furnace, a batch furnace, a bell furnace, a pit furnace operating at atmospheric pressure. 